1. Field of the Invention
The invention relates generally to silicon micromachining, and in particular to micromachining torsion hinges that couple for relative rotation two members such as those included in optical beam vibratory scanners.
2. Description of the Prior Art
Beam scanners are used in digital imaging, printing, bar code readers, optical reading and writing systems, surface inspection devices and various scientific and industrial implements. Such scanners deflect a beam of light, usually from a fixed source, over an angle ranging from several degrees to tens of degrees. The beam sweeps back and forth at a frequency determined in part by the mirror resonant frequency. A typical vibrational scanner of the prior art is described in U.S. Pat. No. 4,732,440 to J. Gadhok. The idea of making torsional scanners within a silicon body was proposed at an early date by K. Peterson, Proc. IEEE, vol. 70, no. 5, p. 61, May 1982. See also U.S. Pat. No. 4,317,611 to K. Peterson.
FIG. 1, depicting a scanner shown in FIG. 39 of Peterson, Proc. IEEE, supra, p. 61, includes a micromachined torsional mirror 11, supported by torsion bars 13 and 15 within silicon body 17 (xe2x80x9cmicro scannerxe2x80x9d hereafter). The aforementioned article describes typical mirror parameters, such as the modulus of silicon, the typical wafer thickness, the length of the torsion bar and the dimensions of the mirror. The width of the torsion bars is on the order of 500 micrometers, while the length of the torsion bars is approximately 0.2 centimeters. The mirror is approximately 0.22 centimeters on a side. The cut which isolates the mirror from the silicon body and also defines the torsion bars is approximately 0.02 centimeters in thickness. Each cut is made by anisotropically etching the silicon. The silicon body rests on glass substrate 21 which has vapor deposited electrodes 23 and 25. A depression 27 is etched into the glass to receive silicon body 17 which rests on a linear support ridge 29. A high voltage is applied first to one electrode then the other in a continuing out-of-phase sequence from a drive circuit. The electric field generated by the electrodes tilts the mirror first to one side and then the other. The restoring force of the torsion bars works against each deflection. The resonant frequency of the mirror can be calculated with well known formulas cited in the above-mentioned articles, although air damping creates an error in the resonance frequency. The substrate, electrodes and drive circuit are part of the micro scanner.
Two dimensional micromachined silicon flexure structures, used as gyroscopes, are known in the art. See U.S. Pat. No. 5,016,072 to P. Greiff. Such structures are similar to micro scanners in construction and vibratory characteristics.
One of the problems encountered in the prior art is in restricting vibrations to a single desired torsional mode. An object of the invention was to devise a micro scanner which vibrates at a single desired mode of vibration and to be self-oscillating at its natural fundamental frequency. Another difficulty with the prior art structures and fabrication methods is an inability to control, balance, or eliminate stress in micromachined plates or frames. Yet another difficulty encountered in fabricating micro scanners is obtaining very high quality mirrors and torsion bars that have a specified thickness and, and, if desired, that are extremely thin.
U.S. Pat. No. 4,365,863 entitled xe2x80x9cOptical Switch for a Very Large Number of Channelsxe2x80x9d that issued Dec. 28, 1982, on an application filed by Georges J. G. Broussaud (xe2x80x9cthe Broussaud patentxe2x80x9d) discloses an optical switching system that has two optical fiber arrays which face one another. In the space between these two arrays, a pair of deflectors angularly redirect a beam of light emitted by an optical fiber in one of the arrays through free space and into a selected one of the many optical fibers in the facing array. This patent discloses that the deflectors may be provided by an optical-mechanical device that operates on the principle of the diasporameter. The Broussaud patent also discloses that pairs of acousto-optical deflectors, that deflect light by photon-phonon interaction within a crystal medium, arranged in a cross configuration may also provide the deflector.
A paper by Kari Gustafsson and Bertil Mxc3x4k entitled xe2x80x9cFiberoptic Switching And Multiplexing With a Micromechanical Scanning Mirrorxe2x80x9d published at pages 212-215 of the describes a fiberoptic switch and multiplexer. This fiberoptic switch and multiplexer uses micromechanical mirrors vibrating torsionally at a resonant frequency of approximately 40 hKz to repetitively couple light periodically between a pair of optical fibers. If the torsionally vibrating mirrors disclosed in this paper could, instead, be held stationary at a fixed particular angle, and if instead of rotating about a single axis such mirrors could rotate through a sufficiently large angle about a pair of non-parallel axes, then micromechanical mirrors could be substituted for the optical-mechanical or acousto-optical deflectors described in the Broussaud patent.
An object of the present invention is to provide a method of operating a micromechanical structure which includes a dynamic member that can rotate either about a single axis, or about a pair of non-parallel axes, in which the dynamic member is held stationary at a fixed particular angle.
Another object of the present invention is to provide a method of operating a micromechanical mirror for reflecting a beam of light that can rotate either about a single axis, or about a pair of non-parallel axes, in which the mirror is held stationary at a fixed particular angle.
Briefly, the present invention is a method for operating an integrated, micromachined structure that includes providing a micromachined structure that is monolithically fabricated from a stress-free semiconductor layer of a silicon substrate. In one embodiment of the method the micromachined structure includes a reference member, a first pair of torsion hinges projecting from the reference member, and a first dynamic member that is coupled only by the first pair of torsion hinges to the reference member. The first pair of torsion hinges supports from the reference member in a first rest position with respect to the reference member if no external force is applied to the first dynamic member. The first pair of torsion hinges alone also support the first dynamic member for rotation about a first axis with respect to the reference member. The first dynamic member, as supported only by the first pair of torsion hinges, exhibits a plurality of vibrational modes with respect to the reference member including a principal torsional vibrational mode of rotation about the first axis, a vertical shaking vibrational mode, a vertical rocking vibrational mode, a lateral shaking vibrational mode, and a lateral rocking vibrational mode. Each vibrational mode of the first dynamic member has a vibrational frequency.
In this particular embodiment of the method the micromachined structure the first dynamic member includes a frame to which the first pair of torsion hinges couple, and from which project a second pair of torsion hinges that are oriented non-parallel to the first pair of torsion hinges. The first dynamic member of this particular embodiment of the method also includes a second dynamic member that is coupled only by the second pair of torsion hinges to the frame. The second pair of torsion hinges support the first dynamic member in a second rest position with respect to the frame if no external force is applied to the second dynamic member. The second pair of torsion hinges, the frame, and the first pair of torsion hinges alone also support the second dynamic member for rotation about a second axis with respect to the frame and about the first axis with respect to the reference member. The second dynamic member, as supported only by the second pair of torsion hinges, the frame, and the first pair of torsion hinges, exhibits a plurality of vibrational modes with respect to the frame including a principal torsional vibrational mode of rotation about the second axis, a vertical shaking vibrational mode, a vertical rocking vibrational mode, a lateral shaking vibrational mode, and a lateral rocking vibrational mode. Each vibrational mode of the second dynamic member has a vibrational frequency.
The method also includes applying a first force to the first dynamic member that urges the first dynamic member to rotate about the first axis out of the first rest position to a fixed particular angle with respect to the reference member. The method further includes applying a second force to the second dynamic member that urges the second dynamic member to rotate about the second axis out of the second rest position to a fixed particular angle with respect to the frame.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.